The invention relates generally to the field of optical communications. In particular, the invention relates to a variable-bit-rate optical communication system and to methods of operating a variable-bit rate optical communication system with nearly optimum filtering.
Conventional communication systems are typically peak power limited or based on peak power limited designs. In operation, these systems maximize transmitted power by modifying the pulse width for each particular bit rate while maintaining a constant duty cycle.
FIG. 1 illustrates a prior art method of increasing the energy-per-bit in a peak power limited multi-rate communication system. In this prior art method, the pulse width changes in proportion to the repetition rate so that the duty cycle remains at a constant value. The maximum peak output power is limited regardless of the average power transmitted. Consequently, it is advantageous to use modulation formats that maximize the power on-to-off duty cycle. In the method of FIG. 1, the duty cycle is 50%, which is typical for on-off key (OOK) communication systems. The energy-per-bit is increased by increasing the period from 0.1 to 1.0.
It is difficult to achieve optimum performance with peak power limited multi-rate communication systems. In order to achieve optimum or matched performance in these prior art peak power limited multi-rate communication systems, the receivers of these systems must have a sinc( ) transfer function (i.e. the Fourier transform of the transmitted rectangular pulse). Presently, optical sinc( ) filters are not commercially available. Furthermore, the receivers must have a different receiver filter for each bit rate to allow the receiver to remain matched to each specific transmitted pulse shape. Using different filters for each bit rate would greatly increase the cost and complexity of the multi-rate communication system.
It is therefore an object of this invention to achieve optimum or matched performance in a variable-rate communication system. It is another object of this invention to provide a variable-rate communication system having bandwidth-on-demand or fall-back modes for communications over a noisy or uncertain channel. It is another object of this invention to provide an optical filter that generates a double pass Fabry-Perot transfer function.
It is another object of this invention to provide a method of reducing a bit error rate of a digital communication system operating in a noisy channel. It is another object of this invention to provide a method of reducing intersymbol interference in a communication system.
It is another object of this invention to provide a method of optimizing variable-bit-rate communications. It is another object of this invention to provide a method of performing optimal communications at one or multiple data rates using spectral or symmetric filtering. It is another object of this invention to provide a method of optimizing a modulator extinction ratio in a communication system.
The present invention applies to all types of communication systems including RF, microwave, and optical systems. The present invention applies to communication systems operating at one or multiple data rates.
A discovery of the present invention is that multi-rate communications can be efficiently achieved in a communication system that employs average power limited amplifiers, such as an Erbium-Doped Fiber Amplifier (EDFA), operating in saturation.
Another discovery of the present invention is that near optimum (matched) filtering can be achieved in a communication system employing an Average Power Limited (APL) amplifier operating in saturation and employing spectral or symmetric filtering. In such systems, the pulse shape of the communication signal can be adjusted prior to the APL amplifier so that the transmitted signal is matched to the receiver thereby maximizing the received energy-per-bit, without sacrificing transmitted power.
Another discovery of the present invention is that a variable rate communication system employing an average power limited amplifier operating in saturation and employing PPM signaling can provide near quantum limited performance.
In one embodiment, the present invention features a variable-bit-rate communication system that includes a variable-bit-rate transmitter which generates digital data having a bit rate. The digital data comprises a sequence of signaling waveforms having a duty cycle, where each signaling waveform has the same shape. The sequence of signaling waveforms is transmitted across a channel. The channel may be any communication channel such as free space or a fiber channel.
In another embodiment, the present invention features a variable-bit-rate communication system that includes a variable-bit-rate transmitter which generates digital data having at least a first and a second bit rate. The digital data comprises a sequence of signaling waveforms having at least a first and a second duty cycle, respectively, where each signaling waveform has the same shape. The amplified sequence of signaling waveforms is transmitted across a channel. The channel may be any communication channel such as free space or a fiber channel.
In one embodiment, the transmitter is substantially average power limited. The transmitter includes an optical average power limited amplifier that in one embodiment is an Erbium-doped fiber amplifier. The optical average power limited amplifier is operated in saturation so that each signaling waveform has a maximum power that is determined by an average power limit of the amplifier and the duty cycle of the waveform. The amplitude of each signaling waveform is inversely proportional to its duty cycle.
A variable-bit-rate receiver receives the digital data generated by the transmitter and transmitted across the channel. The receiver typically includes an optical pre-amplifier that amplifies the digital data. The receiver also includes a receiver filter that in various embodiments may be a Fabry-Perot, interference, Bragg grating, or a multi-pass optical filter such as a multi-pass Fabry-Perot optical filter. In one embodiment, the receiver filter has a transfer function that is substantially equal to the net transmitter transfer function (or the conjugate match of the net transmitter transfer function) so as to spectrally and temporally match the transmitter and the receiver. A detector detects digital data received by the receiver.